The invention relates to a process for making crosslinked polyvinylpyrrolidone. In particular, the invention is a process for reducing the toxicity and volume of aqueous waste generated in a base-catalyzed process for making the polymer.
Polyvinylpyrrolidones have diverse utility. They are used in polymer films, adhesives, hair and skin-care formulations, pharmaceutical tablet binders and disintegrants, and beverage clarifiers. Polyvinylpyrrolidones are normally produced by free-radical or base-catalyzed polymerization of N-vinylpyrrolidone (NVP).
Free-radical initiators, such as hydrogen peroxide or organic peroxides, polymerize NVP to give polymers having relatively low molecular weight and a low degree of crosslinking. These products (e.g., xe2x80x9cPVP K30xe2x80x9d and xe2x80x9cPVP K90xe2x80x9d) are soluble in water and alcohols, and they can be purified by treating their solutions with adsorbants or ion-exchange resins (see, e.g., U.S. Pat. No. 4,795,802).
In contrast, crosslinked polyvinylpyrrolidone (xe2x80x9ccrosslinked PVPxe2x80x9d or xe2x80x9cPVP-Pxe2x80x9d) has a high molecular weight and a high degree of crosslinking. It is produced by base-catalyzed polymerization of NVP. Crosslinked PVP is usually produced by one of two general methods. In one approach, NVP is polymerized in the presence of an added difunctional crosslinker. In another approach, the crosslinker is generated xe2x80x9cin situxe2x80x9d in a two-stage process. In the first stage, an aqueous mixture containing N-vinylpyrrolidone (NVP) and about 0.4 to 0.8 wt. %, based on the amount of NVP, of an alkali metal hydroxide (usually NaOH) is heated to about 140xc2x0 C. to generate divinyl crosslinkers. After several hours, the mixture is cooled to about 100xc2x0 C., and polymerization begins.
Crosslinked PVP is not soluble in water or alcohols. Therefore, impurities cannot be removed by forming a solution and treating it with, for example, an ion-exchange resin or activated carbon. Instead, a typical workup for PVP-P starts with extensive water washing to remove residual alkali metal hydroxide residues. Usually, the polymer is washed until the pH of the washings is close to 7. This is followed by washing with aqueous acid to neutralize base and convert residual NVP to the less-toxic 2-pyrrolidone. A final water wash is then used to remove traces of acid from the PVP-P.
A large volume of water is needed in the three steps to purify the polymer, so a lot of wastewater is generated. Consequently, a PVP-P manufacturer has high disposal costs. Moreover, because the waste-water from the initial washing step normally contains a high concentration (100-1000 ppm) of NVP, the manufacturer must find an environmentally acceptable way to dispose of this relatively toxic waste stream.
U.S. Pat. No. 5,239,053 teaches a process for purifying vinyl lactam polymers, including crosslinked and linear (uncrosslinked) PVP. The reference does not deal with issues of waste volume or toxicity. Residual NVP is eliminated by treatment with an acid or carbon dioxide. In the examples that show how to treat crosslinked PVP (see Examples 1, 9, 10, and 12), the polymer samples are first washed several times with water and are then xe2x80x9creconstitutedxe2x80x9d with water to give an aqueous mixture having an approximately neutral pH. These washing steps, which are performed prior to any acid or carbon dioxide treatment, generate an aqueous waste stream that contains a substantial amount of NVP. Ideally, such a waste stream would be avoided.
One way to avoid using large volumes of water is to simply reduce the amount of water in the aqueous wash solutions. Another possible solution is to skip water washing and use only aqueous acid (to remove NVP) followed by aqueous NaOH (to neutralize acid). Unfortunately, these approaches usually give PVP-P that does not meet at least one of the important product specifications. Crosslinked PVP used in beverage clarification, for example, requires a neutral polymer having residual NVP less than 5 ppm and residual Na less than 250 ppm.
In sum, the industry would benefit from improved ways of making crosslinked PVP. In particular, a process for making PVP-P that generates a reduced amount of aqueous waste is desirable. A process that produces aqueous waste streams that contain little or no N-vinylpyrrolidone is especially needed. Ideally, the process would give crosslinked PVP that meets or exceeds important product specifications.
The invention is a four-step process for making crosslinked PVP. First, an aqueous mixture that contains N-vinylpyrrolidone and an alkali metal hydroxide is heated in a sealed reactor under added pressure to generate a crosslinker. The reactor temperature is then reduced to initiate polymerization and produce a mixture that contains crosslinked polyvinylpyrrolidone (PVP-P) and residual N-vinylpyrrolidone. Water is added, and the resulting aqueous PVP-P mixture is heated in the presence of a protic acid at pH less than 4 to eliminate NVP. Finally, the PVP-P is neutralized with aqueous alkali metal hydroxide.
By using the process described above, we reduced the volume of aqueous waste generated to less than about 20 L/kg of PVP-P produced. Moreover, the purified PVP-P has residual NVP less than 5 ppm and residual alkali metal content less than 250 ppm. Importantly, none of the aqueous waste generated has a residual NVP concentration greater than about 10 ppm.
The process of the invention gives high-quality crosslinked polyvinylpyrrolidone (PVP-P) in four steps while generating a reduced quantity of aqueous waste having relatively low toxicity.
In step one, a crosslinker is generated in situ. An aqueous mixture that contains from about 70 to about 90 wt. %, preferably from about 75 to about 85 wt. %, of N-vinylpyrrolidone (NVP) is heated in the presence of an alkali metal hydroxide. Suitable alkali metal hydroxides include sodium hydroxide, potassium hydroxide, lithium hydroxide, and the like. Sodium hydroxide is particularly preferred. The amount of alkali metal hydroxide used in this step is preferably less than about 1.0 mole %, more preferably less than about 0.7 mole %, based on the amount of N-vinylpyrrolidone used. This amount is relatively low compared with the amount generally used, which is typically 1.5 to 2.5 mole %. For example, a typical amount of sodium hydroxide used in the industry is about 0.6 wt. % (about 2 mole %).
The ability to use a low concentration of alkali metal hydroxide in step one is an advantage of the invention because it facilitates the preparation of PVP-P that meets product specifications for residual NVP and residual alkali metal content. There is a xe2x80x9cdomino effectxe2x80x9d here: the less alkali metal hydroxide used in step one, the less acid needed for step three, and consequently, the less base needed for neutralization step 4, and the less residual alkali metal in the PVP-P.
The aqueous NVP and alkali metal hydroxide are heated at a temperature within the range of about 130xc2x0 C. to about 150xc2x0 C. to generate the crosslinker. A more preferred range is from about 135xc2x0 C. to about 145xc2x0 C.; most preferred is about 140xc2x0 C. As discussed above, the idea of generating a crosslinker in situ prior to polymerization of NVP is well known, but it is normally generated at higher base concentrations.
Step one is performed under added pressure, preferably at least about 40 psig, more preferably at least about 50 psig. This is conveniently done by sealing the reaction vessel (ideally an autoclave reactor or the like) and pressurizing to at least about 40 psig prior to heating. A similar approach is described in WO 94/20555, which teaches that elevating the initial reactor pressure to at least 2 bars (about 29 psig) reduces the xe2x80x9cinduction time,xe2x80x9d i.e., the amount of time needed for polymerization to begin once the temperature is dropped to about 100xc2x0 C.
Most of the NVP polymerizes in step two. The reaction mixture from step one is simply cooled (or allowed to cool) to a temperature within the range of about 95xc2x0 C. to about 105xc2x0 C., preferably from about 98xc2x0 C. to about 102xc2x0 C., to initiate xe2x80x9cpopcornxe2x80x9d polymerization (see, e.g. WO 94/20555). As suggested in the previous paragraph, an induction time precedes the onset of polymerization. Once polymerization begins, an exotherm is normally observed. The polymerization is usually complete within about 5 hours. The product from step two is crosslinked polyvinylpyrrolidone that contains up to about 4.0 wt. % (40,000 ppm) of residual NVP.
Step three involves acid treatment of the PVP-P. First, water is added to the crosslinked PVP product. The amount of water used is an amount needed to give an easily stirred aqueous suspension. Typically, an amount within the range of about 4 to about 12 liters of water per kilogram of PVP-P, preferably from about 6 to about 10 L/kg, will suffice. The aqueous PVP-P mixture is heated to a temperature within the range of about 80xc2x0 C. to about 100xc2x0 C., more preferably from about 85xc2x0 C. to about 95xc2x0 C., most preferably to about 90xc2x0 C. A protic acid is added, preferably a little at a time, until the pH of the mixture is less than about 4.
Suitable protic acids include acetic acid, formic acid, propionic acid, hydrochloric acid, phosphoric acid, sulfuric acid, and the like. Formic acid and acetic acid are particularly preferred.
Using enough protic acid to reach a pH less than about 4 is important for reducing the residual NVP to an ultimate target specification of less than 5 ppm. We found that residual NVP in the polymer is too high if the pH is reduced to a value greater than 4 (see Comparative Examples 4 and 5).
Heating to at least about 80xc2x0 C. during acid treatment is also important for reducing residual NVP to acceptable levels. Treatment with acetic or formic acid at 90xc2x0 C. provides PVP-P having residual NVP less than 5 ppm. In contrast, we found that either room temperature acid treatment (Comparative Example 6) or acid treatment at 70xc2x0 C. (Comparative Example 7) is ineffective in eliminating NVP.
In step four, the PVP-P is neutralized with aqueous alkali metal hydroxide. Neutralization can be accomplished by any suitable method. In one convenient approach, the aqueous, acidic PVP-P mixture from step three is filtered to isolate the solids, the solids are slurried in water, and aqueous alkali metal hydroxide solution is added until the pH reaches a targeted value. Preferably, enough alkali metal hydroxide is added to raise the mixture pH to greater than 5, more preferably to a pH within the range of about 5.5 to 6.0. The amount of alkali metal hydroxide used in step four should be minimized to help in keeping the alkali metal content of the final PVP-P within specifications. The neutralized product is isolated, preferably by filtration, and is usually dried under vacuum. Example 1 illustrates this neutralization approach.
In another convenient neutralization method, the aqueous, acidic PVP-P mixture is not filtered. Instead, the aqueous alkali metal hydroxide is added directly to the acidic mixture (xe2x80x9cdirect neutralizationxe2x80x9d) until the pH reaches the targeted value, which is again preferably greater than 5, and more preferably within the range of about 5.5 to 6.0. When direct neutralization is used, it is usually necessary to wash the PVP-P with water to remove alkali metal hydroxide residues.
As shown in Example 10, direct neutralization, followed by filtration and water washing gives a PVP-P product that meets the target specification ( less than 250 ppm) for sodium content. In contrast, when the water washing step is omitted, the PVP-P usually contains too much alkali metal (see Comparative Examples 11-13; residual Na=1200, 730, or 1600 ppm).
The process of the invention generates a reduced amount of aqueous waste compared with conventional processes, which normally utilize multiple water washes prior to any acid treatment or neutralization steps. In particular, the process of the invention generates less than or equal to about 20 liters of wastewater per kilogram of PVP-P produced, and preferably less than 15 L/kg.
Importantly, the process generates an overall aqueous waste stream having low toxicity. Conventional approaches start with multiple water washes and produce a waste stream having 100-1000 ppm of NVP (see, e.g., Example 1 from U.S. Pat. No. 5,239,053 and Comparative Examples A and 3, below). In contrast, the process of the invention starts with acid treatment to convert NVP to less-toxic hydrolysis products, so none of the aqueous waste streams contains more than about 10 ppm, usually not more than about 5 ppm, of residual NVP.
Finally, the process of the invention affords crosslinked polyvinylpyrrolidone that meets important product specifications. In particular, the residual NVP in the polymer is less than 5 ppm, preferably less than 1 ppm. The residual alkali metal content is less than 250 ppm, preferably less than 100 ppm. With conventional methods, it is difficult to produce on-spec PVP-P while generating a reduced volume of low-toxicity aqueous waste.